From 3bf0dbf0fcd8e8164ee80120720b94ac513cfbc9 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Fri, 27 Mar 2026 10:04:37 -0400
Subject: [PATCH 1/3] Do not interpolate non string values automatically

---
 .../src/config/config-interpolation.ts        |  5 ++--
 .../test/config/config-interpolation.test.ts  | 26 +++++++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/packages/compiler/src/config/config-interpolation.ts b/packages/compiler/src/config/config-interpolation.ts
index d96d8c4dd15..a0fe5364481 100644
--- a/packages/compiler/src/config/config-interpolation.ts
+++ b/packages/compiler/src/config/config-interpolation.ts
@@ -114,8 +114,9 @@ export function resolveValues<T extends Record<string, unknown>>(
       resolvingValues.delete(keyPath);
       return value;
     }
-    const replaced = value.replace(VariableInterpolationRegex, (_, expression) => {
-      return (resolveExpression(expression) as string) ?? `{${expression}}`;
+    const replaced = value.replace(VariableInterpolationRegex, (match, expression) => {
+      const resolved = resolveExpression(expression);
+      return typeof resolved === "string" || typeof resolved === "number" || typeof resolved === "boolean" ? String(resolved) : match;
     });
     resolvingValues.delete(keyPath);
     return replaced;
diff --git a/packages/compiler/test/config/config-interpolation.test.ts b/packages/compiler/test/config/config-interpolation.test.ts
index 6a639444ce7..f8c5d711252 100644
--- a/packages/compiler/test/config/config-interpolation.test.ts
+++ b/packages/compiler/test/config/config-interpolation.test.ts
@@ -133,6 +133,32 @@ describe("compiler: config interpolation", () => {
         },
       ]);
     });
+
+    it("does not interpolate variables that resolve to non-string values", () => {
+      const [resolved, diagnostics] = resolveValues({
+        "output-file": "openapi.{file-type}",
+        "file-type": ["yaml", "json"] as any,
+      });
+      expectDiagnosticEmpty(diagnostics);
+      deepStrictEqual(resolved, {
+        "output-file": "openapi.{file-type}",
+        "file-type": ["yaml", "json"],
+      });
+    });
+
+    it("interpolates number and boolean values", () => {
+      const [resolved, diagnostics] = resolveValues({
+        path: "v{version}/debug-{debug}",
+        version: 3 as any,
+        debug: true as any,
+      });
+      expectDiagnosticEmpty(diagnostics);
+      deepStrictEqual(resolved, {
+        path: "v3/debug-true",
+        version: 3,
+        debug: true,
+      });
+    });
   });
 
   describe("expandConfigVariables", () => {

From cab514195d345f0cacd44ccd9d47d6d68cf65188 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Fri, 27 Mar 2026 10:11:04 -0400
Subject: [PATCH 2/3] chg

---
 .../fix-file-type-interpolation-2026-2-27-10-9-43.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)
 create mode 100644 .chronus/changes/fix-file-type-interpolation-2026-2-27-10-9-43.md

diff --git a/.chronus/changes/fix-file-type-interpolation-2026-2-27-10-9-43.md b/.chronus/changes/fix-file-type-interpolation-2026-2-27-10-9-43.md
new file mode 100644
index 00000000000..081e574e862
--- /dev/null
+++ b/.chronus/changes/fix-file-type-interpolation-2026-2-27-10-9-43.md
@@ -0,0 +1,12 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Do not interpolate non primitive values in config automatically
+  ```yaml
+      file-type: ["json", "yaml"]
+      output-file: "openapi.{file-type}"
+  ```
+  Will not be interpolated as `openapi.json,yaml` but keep the placeholder `{file-type}` intact for the emitter to handle.

From 590e8157005e5db58e35b1f4209fb0f8c5126ea2 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Fri, 27 Mar 2026 10:32:37 -0400
Subject: [PATCH 3/3] format

---
 packages/compiler/src/config/config-interpolation.ts | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/packages/compiler/src/config/config-interpolation.ts b/packages/compiler/src/config/config-interpolation.ts
index a0fe5364481..705cf2bce24 100644
--- a/packages/compiler/src/config/config-interpolation.ts
+++ b/packages/compiler/src/config/config-interpolation.ts
@@ -116,7 +116,11 @@ export function resolveValues<T extends Record<string, unknown>>(
     }
     const replaced = value.replace(VariableInterpolationRegex, (match, expression) => {
       const resolved = resolveExpression(expression);
-      return typeof resolved === "string" || typeof resolved === "number" || typeof resolved === "boolean" ? String(resolved) : match;
+      return typeof resolved === "string" ||
+        typeof resolved === "number" ||
+        typeof resolved === "boolean"
+        ? String(resolved)
+        : match;
     });
     resolvingValues.delete(keyPath);
     return replaced;